In non-destructive testing of tubular metal products, two techniques are employed by a vast number of testing systems. These two techniques are eddy-current testing and remote-field testing. While these techniques are often confused with each other, particularly by those familiar with only one of the two techniques, they are vastly different from each other.
In eddy-current inspection systems, a magnetic field is imposed upon the tubular metal product under test. This magnetic field generates eddy currents within the tubular metal product. Cracks or other flaws in the tubular metal product may produce disturbances in these eddy currents. These disturbances may then be detected by the appropriate sensors.
Ferromagnetic materials, e.g., steel and iron, tend to rapidly swamp out eddy currents due to the formation of strong secondary magnetic fields. Therefore, eddy-current systems are best suited for tubular metal products fabricated of non-ferromagnetic conductive materials, e.g., stainless steel, brass, copper.
With tubular metal products fabricated of non-ferromagnetic materials, the primary magnetic field is confined to the immediate area surrounding the excitation coil(s). This necessitates a close-order detection of the primary magnetic field. Such a close-order detection system would have its sensors proximate where the eddy-current field is strongest. Eddy-current systems often take advantage of this close-order operation by providing both excitation and detection within a single unit.
In addition, eddy-current techniques require that the sensors be in near physical contact, i.e., within 1.25 mm (0.05 inch), of the inner wall of the tubular metal product. This proximity may lead to problems in negotiating curves and/or fittings, and when encountering debris within the tubular metal product. For this reason, eddy-current systems are often limited to the inspection of new (i.e., clean) tubular metal products, especially those have long and substantially straight sections.
Remote-field testing (RFT) inspection systems, on the other hand, depend upon the detection of distortions of the primary magnetic field as it passes through the pipe wall again at the receiving coils at two or more pipe diameters distance. This allows the RFT sensors to detect the primary magnetic field beyond the range of eddy current effects that may interact and produce false errors. RFT systems are therefore best suited for use with tubular metal products fabricated of ferromagnetic materials.
In a typical RFT system, the primary magnetic field is toroidal, with the outer surface of the torus constrained by the ferromagnetic tubular metal product and the inner surface of the torus (i.e., the “hole” well within the inner surface of the tubular metal product). This allows the RFT system to sense distortions in the magnetic field well within the inner surface of the tubular metal product. Therefore, a greater clearance between various units of an RFT system is allowable than with the units of an eddy-current system. This allows for greater negotiability of the tubular metal product through curves and fitting and over debris. RFT systems are therefore better suited for tubular metal products that are not new (i.e., dirty).
A problem exists with RFT inspection systems in that small flaws that are inline with the lines of flux of the primary magnetic field tend to be nearly undetectable. For example, a longitudinal flaw will cause very little perturbation in a longitudinal field.
A weak perturbation, such as that of a longitudinal flaw in a longitudinal field, can be made more apparent by scanning across the field rather than with the field, i.e., scanning latitudinally in a longitudinal field. This poses several electrical and mechanical problems not readily addressed by traditional RFT systems.
Another problem exists with RFT systems in that the detection of the primary magnetic field at a reasonable distance from the excitation coils, in order to be beyond the range of eddy-current effects, results in a relatively weak detection signal. This weak signal is more susceptible to noise contamination than the signals detected by traditional eddy-current systems.
Since a preponderance of tubular metal products are fabricated of ferromagnetic materials, it is desirable that an RFT system be provided that detects latitudinal, longitudinal, and point flaws in a tubular metal product while producing a robust and reliable detection signal.